The Justice Department is expected to oust the head of the Bureau of Alcohol, Tobacco, Firearms and Explosives, according to people familiar with the matter, amid a troubled federal antitrafficking operation that has grown into the agency's biggest scandal in nearly two decades.

Moves toward the replacement of Kenneth Melson, acting ATF director since April 2009, could begin next week, although the precise sequence of events remains to be decided, these people said.